1. Field of Invention
The present invention relates to a high electron mobility transistor (HEMT) and a manufacturing method thereof; particularly, it relates to an enhanced mode HEMT with a vertical channel and manufacturing method thereof.
2. Description of Related Art
FIGS. 1A and 1B show a schematic cross-section view and an energy band diagram of a prior art high electron mobility transistor (HEMT) 100. As shown in FIG. 1A, the HEMT 100 includes a substrate 11, a gallium nitride (GaN) layer 12, an aluminum gallium nitride (AlGaN) layer 14, a gate 15, a source 16, and a drain 17. A two dimensional electron gas (2DEG) 18 is formed at a junction between the GaN layer 12 and the AlGaN layer 14, and the 2DEG 18 is electrically connected to the source 16 and the drain 17. As shown in FIG. 1B, the Fermi level Efs of the GaN layer 12 and the Fermi level Efb of the AlGaN layer 14 are at the same level. The conduction level, i.e., the lowest level of the conduction band, Ecs of the GaN layer 12 and Ecb of the AlGaN layer 14, and the valence level, i.e., the highest level of the valence band, Evs of the GaN layer 12 and Evb of the AlGaN layer 14, are bended at the junction between the GaN layer 12 and the AlGaN layer 14, such that electrons are trapped in the electron well 18a. These trapped electrons can reduce Coulomb scattering to increase the electron mobility in the 2DEG 18, such that the operation speed of the HEMI 100 is faster than a conventional semiconductor device at ON state.
However, the HEMT 100 is a depletion mode device, i.e., the gate voltage of the HEMI 100 is negative during normal operations. In practical applications, it is not convenient to adopt and operate a depletion mode device, especially in high frequency applications. A positive gate voltage of an HEMT during normal operations can decrease the complexity of the circuitry and the manufacturing cost.
FIG. 2 shows a schematic cross-section view of another prior art HEMI 200 which provides a positive gate voltage in a normal operation to solve the aforementioned problem. Referring to FIG. 2, a part of an AlGaN layer 24 beneath a gate 25 of the HEMI 200 is relatively thinner, such that a piezoelectric effect between the part of the AlGaN layer 24 beneath the gate 25 and a GaN layer 22 is different from that between the other part of the AlGaN layer 24 and the GaN layer 22, and no 2DEG 28 is formed at a junction between the part of the AlGaN layer 24 beneath the gate 25 and the GaN layer 22 initially. The two separated 2DEGs 28 become one continuous area by applying a predetermined positive voltage on the gate 25. Therefore, the HEMT 200 is normally OFF, and a gate voltage which is applied to the gate 25 of the HEMT 200 is positive during normal operations.
Even though the prior art HEMT 200 has a positive normal operation voltage, it is not easy to embody the HEMT 200 because it requires controlling the thickness of the AlGaN layer 24, which is difficult, and the normal operation voltage of the HEMT 200 cannot be stabilized at a positive constant level. Besides, the prior art HEMT 200 has another drawback of a relatively higher gate leakage current than the other prior art HEMTs.
In view of above, the present invention proposes an enhanced mode HEMT having a vertical channel and a manufacturing method thereof, to overcome the drawbacks in the prior art so that the HEMT may have a broader application range.